1. Field of the Invention
The present invention relates to glycol/water separation. More particularly, the invention relates to a process and a system for recovering glycol from glycol/brine streams produced from oil or natural gas wells.
2. Description of the Related Art
A common problem associated with natural gas production is the formation of hydrates. Hydrates are solid compounds that form as crystals and resemble snow in appearance. They are created by a reaction of natural gas with water, and when formed, they are about 10% hydrocarbon and about 90% water. To prevent plugging of production lines and equipment by hydrates, it is common to inject a hydrate inhibitor into the gas well.
Traditionally, when producing gas offshore, methanol has been used as a hydrate inhibitor because it lowered the freezing point of water vapor and thus prevented hydrate formation in flow lines. The methanol was produced from the well along with the brine and the methanol/brine solution was often disposed of by dumping it into the ocean.
More recently, economic and environmental considerations have forced offshore hydrocarbon producers to consider techniques for recovering hydrate inhibitors from the inhibitor/brine streams. Processes that recover methanol from methanol/brine streams are known to those skilled in the art, however, there are disadvantages to these processes. Particularly, methanol recovery systems generally leave a large portion of the methanol in the brine stream that is lost during disposal. Therefore, the environmental problems associated with disposal of the brine stream continue to exist. Additionally, some methanol is lost along with the vapor phase. Because methanol is lost in the recovery process, additional methanol must be purchased and transported to the offshore platform to make up for the losses.
It has been known to use glycol as a hydrate inhibitor for natural gas streams containing fresh water vapor. Glycol recovery systems are also known to those skilled in the art to remove glycol from the glycol/water streams. Generally, these systems are designed to produce glycol/water streams having between about fifty (50%) and about ninety-five (95%) percent glycol.
As shown in FIG. 1, prior art glycol recover systems primarily consisted of a distillation column 10 in which the glycol was concentrated by distilling off the accompanying water. A natural gas stream containing glycol and water was introduced into a series of separator vessels 12 and 14 where the pressure was reduced to flash off the natural gas. The glycol/water stream was then introduced into distillation column 10 where it was heated by reboiler 16, typically a steam reboiler, to drive the water overhead and concentrate the glycol. The recovered glycol stream produced by this process was approximately ninetynine percent (99%) glycol.
While glycol is an effective hydrate inhibitor for use with natural gas wells, the glycol recovery systems of the prior art are not particularly suited for recovering glycol from glycol/brine solutions produced from the natural gas wells.
One significant problem with the prior art system of FIG. 1 is created by the salt and other solids contained in the glycol/brine streams. Glycol/brine streams produced from natural gas wells typically contain between about forty percent (40%) and about sixty percent (60%) glycol, about sixty percent (60%) and about forty percent (40%) water with about ten percent (10%) to about twenty-five percent (25%) weight percent dissolved salt in the produced water. The distillation process often results in precipitation of the salt that can foul and plug the recovery system.
Additionally, the prior art glycol recovery systems such as shown in FIG. 1 are extremely energy intensive. Distillation column 10 requires a reboiler 16 to provide the heat necessary to drive off the water vapor. The heat duty required by the reboiler 16 is significant, approximately 300 MM BTU""s per hour for a nominal 5,000 barrels per day (xe2x80x9cBPDxe2x80x9d) glycol recovery unit.
Thus, the need exists for an environmentally safe and energy efficient process for recovering hydrate inhibitors that are produced from oil or natural gas wells along with a brine stream. Particularly, the need exists for a process that recovers glycol from glycol/brine streams produced from oil or natural gas wells that is less energy intensive than the prior art systems and is not subject to fouling or plugging problems caused by salt and other solids in the stream. Additionally, the need exists for such a glycol recovery system that can be used on offshore production platforms.
Briefly, the present invention is a process and a system for recovering glycol from a stream of glycol and brine that has been produced from an oil well or a natural gas well. The present invention provides an energy efficient recovery process and system with capability for handling salt and other solids contained in the glycol/brine stream.
The system of the present invention comprises three effect evaporator systems in series. Each effect evaporator system comprises an evaporator, a separator vessel, product pumps, and a solids removal system. Triple effect evaporator systems have been known to those skilled in the art for concentration of other solutions, however, use of such systems to recover glycol from glycol/brine streams is novel. A particularly novel feature of the system of the present invention is the combination of a triple effect evaporator system with solids removal systems. The solids removal systems can be a combination of a hydrocyclone and strainers, a continuous disk centrifuge, or other solids removal systems known to those skilled in the art.
The process of the present invention is a novel process which utilizes the triple effect evaporator system of the present invention to remove salt and other solids as well as excess water, leaving a glycol stream that can be reused as a hydrate inhibitor. The process of the present invention begins by preheating a glycol/brine stream comprising approximately fifty percent (50%) glycol. The stream is then subjected to three evaporation cycles.
The first evaporation cycle comprises introducing the preheated stream into a suppressed boiling point evaporator where the stream is heated under a constant pressure. The stream pressure is then dropped to cause a portion of the water contained in the stream to vaporize or flash. The flashing stream is then introduced into a separator vessel where the water vapor is separated from the remaining liquid stream. The water vapor is removed from the separator and condensed. The remaining liquid glycol/brine stream is then pumped from the separator vessel through a solids removal system where precipitated salts and solids are removed.
The above mentioned steps of introducing the stream into an evaporator for heating under pressure, dropping the stream pressure to cause a flash, separating the remaining liquid stream from the vapor stream, condensing the vapor stream, and pumping the remaining liquid stream through a solids removal system to remove salts and other solids are repeated two additional times. Each time these steps are performed the remaining liquid stream becomes more concentrated with glycol and after the third cycle the finished product is approximately ninety percent (90%) glycol. To maximize the energy efficiency of the process of the present invention, heat energy from the water vapor generated in the third evaporation cycle is used to supply heat for the second evaporation cycle, and the heat energy from the second evaporation cycle is used to heat the first evaporation cycle. Additionally, heat from the finished product glycol stream can be recovered and used during the preheating step.